This invention relates to monitoring and control of heat-treating processes and particularly to the control of carbon potential in the atmosphere of a carburizing furnace.
In typical carburization processes metal workpieces such as steel parts are exposed to a carbon-bearing atmosphere within a furnace. Carbon is transferred from the atmosphere to the surface of the parts and is subsequently transferred to subsurface portions by diffusion, thus hardening and strengthening the parts. To consistently achieve the desired properties in the processed parts, the content of carbon in and below the part surface must be carefully controlled. Since direct continuous measurement of the carbon concentration in the metal at the surface is not feasible, it must be inferred from the furnace atmosphere. The systems have been developed for monitoring the carbon potential of the furnace atmosphere, which is a measure of the carburizing power of the atmosphere and which defines the concentration of carbon present in the surface of a steel part in equilibrium with the furnace atmosphere.
One system for monitoring carbon potential is shown in U.S. Pat. No. 4,049,473 to R. L. Davis, II et al. This system includes a thin wire probe located in a temperature controlled chamber separate from the main furnace wherein parts are carburized. Changes in resistivity of the wire resulting from its carburization and decarburization are used to assess and control carbon potential of the main furnace atmosphere. While probes of this type are useful, they may be subject to inaccuracies due to contamination and oxidation and may also be relatively fragile.
Other known systems for determining carbon potential of a heat-treating atmosphere monitor a single parameter or component of the atmosphere. An empirical correlation between the carbon potential and the monitored parameter is then used to calculate the carbon potential. One parameter which may be monitored in a single component system is dew point of the atmosphere, which may be measured by condensing water from the atmosphere on a cool metal surface. Another parameter is carbon dioxide content which may be measured by an infrared analyzer. Alternatively, oxygen content of the atmosphere may be determined from the oxygen potential measured by a solid electrolyte oxygen concentration cell.
A major drawback of these single-parameter systems is that their use involves an assumption that non-monitored parameters remain constant during processing. If variations occur in certain non-monitored parameters, the single-parameter systems will yield inaccurate values of carbon potential as is indicated on the graphs of FIGS. 1 and 2. These figures show the effects on carbon potential of variations in temperature and carbon monoxide content which may occur in a furnace atmosphere during a heat-treating process. The effects illustrated are for a process designed to attain a carbon potential of one percent at 1700.degree. F. FIG. 1 indicates that significant errors in carbon potential may occur in a single component carbon dioxide control system if carbon monoxide content of the furnace atmosphere deviates from the desired value of 20 percent or if temperature fluctuates from the intended setting. Similar errors, though less severe, are indicated in FIG. 2 for a single component oxygen potential control system.
Accordingly, it is an object of the present invention to provide improved apparatus for monitoring and controlling the atmosphere of a heat-processing furnace.
It is a more particular object of the invention to provide improved apparatus for monitoring the carbon potential of the atmosphere in a carburizing furnace.
It is an object of the invention to provide apparatus for determining the carbon potential in a carburizing furnace wherein the carbon potential is determined from measurements of the temperature of the furnace atmosphere and of the partial pressures of at least two gaseous components of the atmosphere.
It is also an object of the invention to provide apparatus for controlling the carbon potential of the atmosphere of a carburizing furnace.